Received 22 August 2011Accepted 29 September 2011Published online 20 October 2011

Acknowledgments:

The authors thank Cambridge Display Technology (CDT), Ltd., for supplying F8BT. This work was supported by the Mid-career Researcher Program (2010-0027764), the National Research Laboratory Program (R0A-2008-000-20057-0), the World Class University (WCU) program (R32-2008-000-10051-0, R31-2008-000-20004-0), and the National Research Foundation of Korea (2010-0028791, 2009-0071834).